The invention relates to a measuring arrangement for an internal combustion engine equipped with injection valves. The measuring arrangement detects the beginning of the injection with an inductive position transducer responsive to the nozzle pin. The position transducer is controlled by a current source.
Inductive sensors are known from DE-OS No. 30 32 381 which detect the time changes of an inductive value. FIG. 4 of this publication illustrates a constant current source connected in series with a variable inductance and an inverter is connected to the node connecting the current source to the inductance. After the output signal of the inverter is processed, a comparator detects the beginning of injection.
Current sources used in this manner are disclosed, for example, in the book entitled "Halbleiterschaltungstechnik" of Tietze-Schenk, 4th Edition, page 53. It has been shown especially advantageous to utilize a zener diode for setting the base voltage of the transistor so that the drive of the transistor is substantially independent of small variations in supply voltage.
However, in the relatively rough environment of a motorized vehicle, disadvantages with respect to the known arrangement have become manifest and are discussed below.
This current source operates in an unsatisfactory manner as a consequence of temperature drift of all components of the current source, and especially by low battery voltage as is the case, for example, during the time the motor is started. This causes the transistor of the current source to become fully conductive when the battery voltage falls off to lower values, and all of the noise signals on the supply voltage are evaluated as operating signals.
A further disadvantage is that the amplitudes of the operating signals exhibit a dependency on rotational speed. Because of this, it becomes necessary to equip the comparators used for digital signal processing with rotational speed dependent thresholds in order to prevent larger errors in fixing the time at which fuel injection is to begin. Comparators equipped with such rotational speed dependent thresholds are disclosed, for example, in DE-OS No. 24 49 836.